Comment 1 for bug 1806344

I'm not sure I understand the issue.

https://docs.openstack.org/nova/latest/configuration/config.html#DEFAULT.default_availability_zone

is the default AZ for a compute service. If the server is created without specifying an availability zone, it can be scheduled to any host and migrated to any other host across AZs.

The only time the server is restricted to a certain AZ is:

1. The user requested a specific AZ when creating the server (and the user should not request "nova", see the warning here: https://docs.openstack.org/nova/latest/user/aggregates.html#availability-zones-azs).

2. The default_schedule_zone https://docs.openstack.org/nova/latest/configuration/config.html#DEFAULT.default_schedule_zone option is set.

It sounds like what you want is #2 so that VMs which are created without a specific AZ being requested will go into a default AZ. If so, then configure that option in nova-api.